The present invention relates generally to solid-state devices for area illumination and, in particular, to an improved light emitting diode lamp employing quantum dot phosphors.
There is considerable interest in developing improved solid-state lamps for area illumination in order to reduce electrical power usage and, ultimately, the production of atmospheric carbon.
Current incandescent lamps, which generally provide black body radiation from a heated filament at approximately 3000° K., produce a continuous spectrum analogous to that of sunlight (white light) with an efficiency of approximately 5%. Sunlight, in contrast, approximates a black body radiator at 5700° K.
Improved efficiencies over incandescent lamps may be obtained by fluorescent lamps which employ a gas discharge to generate ultraviolet light which in turn excites a phosphor coated on the inside of the lamp tube. Such fluorescent lamps reach an efficiency of approximately 20% but provide a spectrum that deviates substantially from the desirable continuous spectrum provided by incandescent bulbs. The spectrum of a fluorescent lamp typically exhibits several distinct spectral peaks associated with each different phosphor used for the coating. A combination of phosphors can produce a spectrum that nevertheless approximates the color perception provided by sunlight.
Light emitting diodes are a promising source for area illumination and currently provide efficiencies ranging from 15 to 30%. The spectral output of an LED is also relatively narrow, defined by the energy released when electrons cross a band gap in the semiconductor materials. Accordingly, broad-spectrum white light requires multiple LEDs (red and green and blue) or, more conventionally, a blue LED illuminating a phosphor similar to the type used in fluorescent lamps. Commonly, a single “yellow” phosphor is used which captures some of the blue light from the LED to down-convert that blue light to yellow. The combined blue and yellow emissions appear to the eye to be approximately white. The resulting light, however, does not match the continuous spectrum normally associate with white light, being particularly deficient in green. For this reason, current white light LEDs can create unnatural color perception that may limit their popularity.
Ideally a “phosphor” could be developed for LEDs that would more closely approximate the continuous spectrum provided by natural sunlight, in particular the blue-green region of the spectrum from 475 to 525 nm, while providing improved efficiencies. Recently some work has suggested that improved efficiencies and light characteristics may be obtained by forming phosphors into quantum dots. In some cases, these phosphors employ quantum dots using relatively expensive or highly toxic materials.